N-Oxoammonium salt

N-Oxoammonium salts are a class of organic compounds with the formula [R1R2N+=O]X−. The cation [R1R2N+=O] is of interest for the dehydrogenation of alcohols. Oxoammonium salts are diamagnetic, whereas the nitroxide has a doublet ground state. A prominent nitroxide is prepared by oxidation of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, commonly referred to as [TEMPO]+. A less expensive analogue is Bobbitt's salt.

Structure of N-oxoammonium salts
The structure of Bobbitt's salt, an N-oxoammonium salt.

Structure and bonding

Oxoammonium cations are isoelectronic with carbonyls and structurally related to aldoximes (hydroxylamines), and aminoxyl (nitroxide) radicals, with which they can interconvert via a series of redox steps. According to X-ray crystallography, the N-O distance in [TEMPO]BF4 is 1.184 Å, 0.1 Å shorter than the N-O distance of 1.284 Å in the charge-neutral TEMPO. Similarly, the N in [TEMPO]+ is nearly planar, but the O moves 0.1765 Å out of the plane in the neutral TEMPO.[1]

The N-oxoammonium salts are used for oxidation of alcohols to carbonyl groups,[2] as well as other forms of oxoammonium-catalyzed oxidations. The nitroxyl TEMPO reacts via its N-oxoammonium salt.[3]

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See also
  • Nitrone – structurally related, the N-oxide of an imine

References
  1. Yonekuta Yasunori, Oyaizu Kenichi, Nishide Hiroyuki (2007). "Structural Implication of Oxoammonium Cations for Reversible Organic One-electron Redox Reaction to Nitroxide Radicals". Chem. Lett. 36: 866–867. doi:10.1246/cl.2007.866.CS1 maint: multiple names: authors list (link)
  2. Bobbitt, James M.; Brückner, Christian; Merbouh, Nabyl (2010). "Oxoammonium- and Nitroxide-Catalyzed Oxidations of Alcohols". Organic Reactions: 103–424. doi:10.1002/0471264180.or074.02.
  3. N. E. Leadbeater, J. M. Bobbitt (2014). "TEMPO-Derived Oxoammonium Salts as Versatile Oxidizing Agents" (PDF). Aldrichchimica Acta. 47 (3).


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